This invention is directed to a system for receiving signals from a passive transponder and, in particular, to a probe antenna and signal processing circuit for transmitting a signal to the passive transponder and receiving the signal transmitted back by the passive transponder.
Transponder and scanner systems are well known in the art. Systems include an interrogator which receives signals from a passive transponder. Such a prior art system is known from U.S. Pat. No. 4,730,188 and includes an antenna which transmits a 400 KHz signal which is received by the transponder. The transponder receives the input signal and outputs a signal. A single antenna and coil is used to both transmit and receive signals. The interrogator receives a divided signal of 40 KHz and 50 KHz. This signal is coded in accordance with a combination of 40 KHz and 50 KHz portions of the transmitted signal to correspond to information stored in a chip contained within the passive transponder.
The prior art interrogator includes an antenna having a single wire wound about a core forming a coil. The coil is used to both transmit and receive signals. The single wire is coupled directly to a driving circuit for producing the interrogation signal transmitted to the passive transponder.
The prior art interrogators have been satisfactory. However, they suffer from the disadvantage that because the coil is being driven directly by an amplifier, it becomes necessary to include load resistors within the driving circuitry. These resistors tended to overheat providing for an inefficient driving system which requires a high voltage input and provides low current output.
The coupling between the interrogator and transponder is through magnetic coupling. It follows that the greater the amperage within the interrogator, the stronger the transmitted magnetic field. The prior art device suffered from the disadvantage that even though high voltage was used a low current resulted reducing the magnetic field generated, thereby reducing the transponder read distance Additionally, the transponder actually outputs a signal at a return frequency which is close to the excitement frequency. The return frequency which is closest to the excitement frequency will be the signal which is highest powered, i.e. has the greatest amplitude. However, because the prior art antennas utilize a single coil they are required to use the lesser frequency return signal of 40 KHz and 50 KHz because the higher frequency return signal and transmitted signal frequency interfere with each other (due to its proximity to the transmitted signal frequency). The 40 KHz/50 KHz signal is transmitted at a lower power thereby reducing read distances. Additionally, because the interrogator operates at 40 KHz, it is subject to background noise interference from television monitoring screens or computer CRTs which by necessity are normally present since they are used in conjunction with microprocessors which are used during scanning. These monitors also operate utilizing a 40 KHz and 50 KHz RF signal. Because these monitors have a high power output relative to the antennas, they interfere with the operation of the interrogator when the interrogator is used in proximity to computers and other various monitors.
Therefore, an efficient system capable of transmitting a signal at one frequency and receiving a signal having a return frequency close to the transmit signal frequency is provided by the instant invention.